feat(webgpu): support transient (memoryless) attachments

[mvaligursky]

https://github.com/gpuweb/gpuweb/blob/main/proposals/transient-attachments.md

Adds support for WebGPU transient ("memoryless") attachments
(GPUTextureUsage.TRANSIENT_ATTACHMENT, shipped in Chrome 146+). On
tile-based GPUs (mobile / Apple Silicon) the driver can keep such an
attachment's contents in on-chip tile memory and skip VRAM allocation
entirely, as its contents only need to live within a single render pass.

This applies to attachments that are cleared on load and discarded on
store, and never sampled, copied or resolved — most notably the MSAA
color resolve-source buffer and depth buffers that are not read back.

Changes:

• Detect support via GraphicsDevice#supportsTransientAttachments.
• Allocate the MSAA color buffer and engine-owned depth buffer with
  TRANSIENT_ATTACHMENT (suppressing COPY_SRC / TEXTURE_BINDING on depth)
  when requested and supported.
• A render-time guard in the WebGPU render target forces clear/discard and
  logs Debug.errorOnce if a transient attachment is ever reloaded or
  stored (e.g. a later pass reuses the target, or a color/depth grab pass),
  preventing a WebGPU validation error.

API Changes:

• createGraphicsDevice options: new transientColor / transientDepth
  booleans (default false), applied to the back-buffer.
• RenderTarget constructor options: new transientColor / transientDepth
  booleans (default false), plus matching read-only getters.
• Both are WebGPU-only hints, silently ignored on WebGL2, where unsupported,
  for single-sampled color, or when an explicit depthBuffer is provided.
  Incompatible with scene color grab (sceneColorMap) and scene depth grab
  (sceneDepthMap) / depth prepass.

Examples:

• render-to-texture: enables transientColor/transientDepth on both the
  off-screen render target and the main back-buffer.

[Copilot]

Pull request overview

Adds WebGPU transient (“memoryless”) attachment support to reduce VRAM allocation for MSAA color buffers and engine-owned depth buffers on tile-based GPUs by using GPUTextureUsage.TRANSIENT_ATTACHMENT, plus runtime guards to enforce clear/discard usage.

Changes:

• Adds transientColor / transientDepth hints on RenderTarget (and createGraphicsDevice options for the backbuffer).
• Allocates eligible WebGPU MSAA color and engine-owned depth textures as transient attachments when supported.
• Adds render-time validation/guardrails (force clear+discard and log once; block depth resolve/grab paths that are incompatible).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/platform/graphics/webgpu/webgpu-render-target.js	Marks attachments as transient and enforces clear/discard ops at render-pass setup time.
src/platform/graphics/webgpu/webgpu-graphics-device.js	Detects transient support; wires backbuffer options; prevents incompatible depth resolve/copy operations.
src/platform/graphics/render-target.js	Introduces transient flags/options and getters on RenderTarget.
src/platform/graphics/graphics-device.js	Adds default init options for transient flags.
src/platform/graphics/graphics-device-create.js	Documents new createGraphicsDevice options.
examples/src/examples/graphics/render-to-texture.example.mjs	Demonstrates enabling transient attachments for backbuffer and offscreen RT.

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